The moving-image-display quality (moving-image quality) of a typical LCD (Liquid Crystal Display) is inferior to that of a CRT (Cathode Ray Tube). This is regarded as a result of slow response speed of the liquid crystal in used.
For the purpose of solving this problem, Journal of the Japanese Liquid Crystal Society (Vol. 3, No. 2, 1999, pp., 99-106) describes an attempt to improve moving-image quality through an increased response speed of liquid crystal, by adopting a Pi-cell structure whereby a Pi-cell is flanked by optical compensators as shown in FIG. 17.
The paper mentions that a Pi-cell shows an improvement in response speed of liquid crystal over a TN liquid crystal cell: namely, a turn-on time of 1 ms and a turn-off time of 5 ms.
The Pi-cell structure successfully yields a response speed that is fast enough to draw an image in a single frame period. However, the moving-image quality of an LCD with a Pi-cell structure is still inferior to that of the CRT. See FIGS. 18 a and 19 a illustrating the moving image display on a CRT and a LCD with a Pi-cell structure respectively. The moving images are supposed to be moving in the directions denoted by the arrows.
The paper attributes the quality differences to illuminating characteristics of the CRT and the LCD. FIG. 18 b shows the “impulse-type” illuminating characteristics of the CRT whereby pixels emit an impulse of light lasting for a short period of time. In contrast, FIG. 19 b shows the “hold-type” illuminating characteristics of the LCD whereby pixels are hold alight continuously. According to the paper, the degradation of moving-image quality occurs in the LCD, because images in successive fields appear overlapping as a result of the motion of viewpoint.
The paper mentions that the problem is solved by the use of a backlight with impulse-type illuminating characteristics similar to those of the CRT. SID (Society for Information Display), 1997, pp., 203-206, “Improving the Moving-Image Quality of TFT-LCDs”, describes a technique to impart impulse-type illuminating characteristics to the LCD (first technique).
According to the first technique, a fluorescent lamp is adopted for use as a backlight of an LCD originally having a hold-type transmittance as shown in FIG. 20 b. The fluorescent lamp is flashed as shown in FIG. 20 c, using a switching circuit for use with a fluorescent lamp configured as shown in FIG. 20 a. The result is impulse-type illuminating characteristics as shown in FIG. 20 d (hereinafter, such an LCD will be referred to as an “entire surface flash type”). The fluorescent lamp in FIG. 20 a exhibits illuminating characteristics as show in FIG. 21 a when a voltage in FIG. 21 b is applied.
The paper describes, as detailed above, a further improvement of moving-image quality of an OCB (Optically Compensated Bend) cell by means of the first technique. A Pi-cell is a type of OCB cell.
The paper further discusses a second technique, whereby the pixels per se of the liquid crystal panel are used as a shutter to impart impulse-type illuminating characteristics to the LCD.
Specifically, a TFT panel 116 is used in which the display section is divided horizontally into an upper screen and a lower screen which are driven by various signals supplied from source drivers 117 and 118 provided to the respective upper and lower screens as shown in FIG. 22 d. 
The upper and lower source drivers 117 and 118 supplies a black signal and a video signal alternately as shown in FIG. 22 a and FIG. 22 c to each pixel of the TFT panel 116. In synchronism with the supply, a gate driver 119 supplies a gate signal shown in FIG. 22 b to the TFTs each constituting a pixel of the TFT panel 116. The result is a blanking signal and a video signal being applied within a field period as shown in FIGS. 23 b to 23 d (hereinafter, such an LCD will be referred to as an “black blanking type”).
According to the second technique, a black display period (interval between RS periods) appears on the hold-type video image in FIG. 23 a, moving from the top to the bottom of the panel as shown in FIGS. 23 b to 23 d. This explains a successful improvement of moving-image quality.
From a viewpoint of flashing a backlight in an LCD module as above, the concept of field sequential color, whereby-a color image display is effected by displaying red, green, and blue images in a time series, is similar to the concept of improving moving-image quality.
SID (Society for Information Display), 1999, DIGEST, pp., 1098-1101, “Field-Sequential-Color LCD Using Switched Organic EL Backlighting” describes a conventional driving method for a field sequential color display. According to the driving method, the device is driven in the time sequence shown in FIG. 24.
Referring to FIG. 24, voltage is applied to a TFT pixel in period (1), response of liquid crystal is awaited in period (2), and an EL (electro-luminescence) backlight is flashed across the screen in period (3). The backlight of this kind of LCD is flashed across the screen similarly to that of the entire-surface-flash-type LCD.
According to the new driving method introduced in the paper, voltage is applied to TFT pixels starting in the top line of the panel and moving down to the bottom line of the panel as shown in FIG. 25. In synchronism with the voltage application to a particular line (however, after a response time of liquid crystal is elapsed), an EL backlight corresponding to that line is flashed.
In prior art example described in the paper, an EL is used as a backlight for use with a field sequential color display; however, a fluorescent lamp may be used instead. In the event, the flashing of the fluorescent lamp should be controlled using the circuit for controlling the flashing of a fluorescent lamp disclosed in Japanese Laid-Open Patent Application No. 11 160675/1999 (Tokukaihei 11 160675; published on Jun. 18, 1999).
FIG. 26 shows the arrangement of a circuit for controlling the flashing of a fluorescent lamp described as a conventional example in the Laid-Open Patent Application.
The circuit for controlling the flashing of a fluorescent lamp, as shown in FIG. 26, includes: high voltage generating means 115 constituted by a DC power source 105 and an inverter 107; and three cold cathode tubes 108, 109, and 110 emitting red, green, and blue light respectively. The cold cathode tubes 108, 109, and 110 are connected in series to switches 111, 112, and 113 respectively. The switches 111 to 113 are each constituted by a high-voltage-resistant bidirectional thyristor which is readily available on the market at a cheap price. By closing one of the switches 111 to 113, a path is established for the high voltage generating means 115 to apply voltage only to the corresponding one of the cold cathode tubes 108 to 110.
This field sequential color technique corresponds to the conventional driving method mentioned above in reference to the SID '99 paper.
However, in a circuit in FIG. 26 disclosed in the Laid-Open Patent Application, the switches 111 to 113 each constituted by a bidirectional thyristor are not resistant enough to high voltage when they are all open; if the high voltage generating means 115 applies voltage, breakdown takes place in one or more of the open cold cathode tubes 108 to 110, disrupting a complete dark state.
To solve this problem, the Laid-Open Patent Application suggests the use of a novel circuit for controlling the flashing a fluorescent lamp which includes high voltage generating means 114 with an additional switch 106 interposed between the DC power source 105 and the inverter 107 as shown in FIG. 27. When no breakdown is desired in any of the three cold cathode tubes 108 to 110, the switch 106 constituting a part of the high voltage generating means 114 is opened to keep the output level of the inverter 107 below a breakdown voltage, preventing breakdown to occur in all of the cold cathode tubes 108 to 110.
A summary prepared for the 1st LCD Forum of the Japanese Liquid Crystal Society, titled “Display Method of Hold-Type Display Device and Quality of Display of Moving Images”, mentions that quality of moving-image displays on a typical LCD is improved effectively by imparting to the LCD illuminating characteristics which are similar to those of the CRT, i.e., impulse-type illuminating characteristics.
The effectiveness of this method is supported by FIG. 28 showing the relationship between flashing ratios (compaction ratio) and five-level average ratings. The flashing ratio is a period during which a backlight or other illuminating means shines divided by a field period of an LCD or another hold-type display. The five levels average rating represents a result of a subjective evaluation of image quality.
For these reasons, the entire surface flash structure and the black blanking structure have been conventionally employed in LCDs to impart illuminating characteristics which are similar to those of impulse types to them.
However, conventional entire-surface-flash- and black-blanking-type displays still have problems as detailed below.
First, in conventional entire surface flash types of LCDs, display scanning is carried out as shown in FIG. 29; therefore, the display period is equal to a backlight flashing period which is given by equation (1):Backlight Flashing Period=Field Period−(TFT Panel Scanning Period+Liquid Crystal Response Period)  (1)
Equation (1) indicates that entire surface flash types of LCDs have a problem such that the backlight flashing period (display period) is reduced by a value equal to the liquid crystal response speed.
Supposing, for example, that the LCD has a Pi-cell structure, a field period is 16.6 ms, and the response time of the liquid crystal (turn-off time of the Pi-cell) is 5 ms, the backlight flashing period of 8.3 ms (equivalent to a 50% flashing ratio in FIG. 28) is only ensured by the scanning period of the TFT panel of 3.3 ms, which is extremely short compared to those of entire surface hold types of LCDs. The TFT panel in an entire-surface-hold-type LCD has a scanning period which is equal to a single field period at 16.6 ms.
Next, in conventional black blanking types of LCDs, display scanning is carried out as shown in FIG. 30; therefore, the display period is given by equation (2):Display Period=Field Period−TFT Panel Scanning Period  (2)
Equation (2) indicates that the display period is independent from the response time of the liquid crystal. Accordingly, in black blanking types, the display period is not affected by the response time of the liquid crystal and is longer than those of entire surface flash types by a value equal to the response time of the liquid crystal.
However, black blanking types of LCDs have a problem in CR (contrast) which is inferior to those of entire surface flash types.
In the following, a comparison is made between black blanking types and entire surface flash types on the CR (contrast) in a field period.
The CR of black blanking types is given by equation (3):CR=(Display Period×Bright Display TransmissionRatio)/(Field Period×Dark Display Transmission Ratio)  (3)
In contrast, the CR of entire surface flash types is given by equation (4):CR=(Backlight Flashing Period×Bright Display Transmission Ratio)/(BacklightFlashing Period×Dark Display Transmission Ratio)  (4)
If, for example, the CRs of a black blanking type of LCD and an entire surface flash type of LCD are obtainable respectively from equations (3) and (4), which are rewritten as equations (5) and (6) when substituting 16.6 ms to the field period, 8.3 ms (equivalent to a 50% flashing ratio in FIG. 28) to the black blanking period, the bright display transmission ratio of the TFT display used of 30%, and the dark display transmission ratio of the TFT display used of 0.1%.CR of Black Blanking Type=(8.3 ms×30 w)/(16.6 ms×0.1%)=150  (5)CR of Entire Surface Flash Type=(8.3 ms×30 w)/(8.3 ms×0.1%)=300  (6)
Equations (5) and (6) indicate that the black blanking type has a lower CR than the entire surface flash type.